(a) Field of the Invention
The present invention relates to a method for preparing a pyrrolidine derivative represented by the formula (1) ##STR2## (wherein R.sup.1 is a hydrogen atom, halogen atom or methyl group; R.sup.2 is a hydrogen atom, alkyl group having 1 to 3 carbon atoms, trifluoromethyl group, haloalkoxy group having 1 to 3 carbon atoms, alkoxy group having 1 to 4 carbon atoms, nitro group, cyano group, phenoxy group, hydroxyl group or halogen atom; X is a hydrogen atom, halogen atom, trifluoromethyl group, alkyl group having 1 to 3 carbon atoms, cyano group or nitro group; and n is 1 or 2 and denotes the number of substituents represented by X, and in the case of n=2, the substituents of X may be identical or different, but R.sup.1, R.sup.2 and Xn are not simultaneously hydrogen).
3,4-trans-4-ethyl-1,3-disubstituted (substituted phenyl)pyrrolidine-2-one represented by this formula has a herbicidal activity.
(b) Description of the Prior Art
It has already been disclosed that certain kinds of pyrrolidine-2-one derivatives have a herbicidal activity (U.S. Pat. No. 4,110,105, EP-A-134,564, U.S. Pat. No. 4,874,422 and U.S. Pat. No. 4,960,457). Thus, 3-chloro-4-chloromethyl-1-(3-trifluoromethylphenyl)pyrrolidine-2-one (general name "flurochloridone") is on the market as a herbicide. In EP 387,869, there are described parts of compounds which can be prepared by the method of the present invention.
These compounds can all be manufactured by a radical cyclizing reaction using tributyltin hydride, and in this case, the pyrrolidine-2-one derivative is obtained as a mixture of a 3,4-trans isomer and a cis isomer. Since the 3,4-trans isomer only has the herbicidal activity, this known method is not preferable as a manufacturing process of the herbicidal compound.
A compound represented by the formula (1) can be obtained by subjecting a compound represented by the formula (2) ##STR3## (wherein R.sup.1, R.sup.2, X and n are as defined above) to a catalytic hydrogenation reaction using palladium, platinum or the like as a catalyst. However, this reaction is not preferable, because when a chlorine atom or bromine atom is present in a substituent on the benzene ring, the elimination reaction of this halogen atom occurs together with the reduction of a double bond a the 3 and 4 positions of 3-pyrroline-2-one and it is very difficult to separate the desired compound.
Among the compounds represented by the formula (1), only compounds in which a steric configuration at the 3 and 4 positions of the pyrrolidine-2-one is in the trans state have the herbicidal activity. In the catalytic hydrogenation reaction using the metallic catalyst, the cis isomer is only obtained. In order to obtain the trans isomer, it is necessary to further carry out isomerization by the use of a base, which involves one more process step. Since this isomerization is an equilibrium reaction, the cis isomer partially remains in the obtained reaction product, and therefore this known technique is unsatisfactory as the preparation method of the compound for the herbicide.